The promise of gene therapy—the replacement of dysfunctional genes with useful ones—has gone largely unfulfilled because the microbial delivery agents used to insert the desirable genes into needy cells haven't been up to the job.
Early in the research, scientists seized on viruses as ideal vectors to deliver genes to patients since these microbes insert their genome into a host cell. However, the agents have proved less than perfect. Viruses can be expensive to prepare and store. Moreover, one of the most promising viruses isn't big enough to tote the large genes required to overcome some troubling diseases.
More serious obstacles have also arisen. Even viruses that scientists have partially disabled sometimes replicate, and the microbes can attract unwanted attention from a patient's immune system.